This invention relates to an air-fuel ratio control system for internal combustion engines, which is capable of appropriately controlling the air-fuel ratio in accordance with the degree of warming-up of the engines.
It has been generally carried out to set the air-fuel ratio of a mixture supplied to an internal combustion engine to such a small value as can secure stable operation of the engine during cold starting of the engine and/or during warming-up of the engine immediately following the cold starting of the engine, and on the other hand, to set the air-fuel ratio to such large values as correspond to operating conditions of the engine after the warming-up of the engine has been completed, since the operation of the engine becomes stable after completion of the warming-up. In order to satisfy required operating characteristics of an internal combustion engine such as driveability, fuel consumption and exhaust emission characteristics at the same time, it is prerequisite that the air-fuel ratio should be accurately controlled in accordance with warming-up conditions of the engine. On the other hand, exhaust gas sensors for sensing the concentration of a specific component in exhaust gases from internal combustion engines, as represented by an O.sub.2 sensor, are generally used in feedback control of the air-fuel ratio. Such exhaust gas sensors need to be activated before they normally operate, that is, the temperature of the sensors per se has to be elevated up to a prescribed activating temperature. Therefore, conventionally, when the engine is started while it is in a cold state, the air-fuel ratio feedback control cannot be effected due to the exhaust gas sensor being inactive.
There have been proposed air-fuel ratio control systems which are intended to appropriately control the air-fuel ratio by taking into account the aforementioned difference in required air-fuel ratio between during cold starting of an internal conbustion engine and after completion of the warming-up of the engine, e.g. by Japanese Patent Publication (Kokoku) No. 57-7297 (hereinafter called "Conventional System 1") and Japanese Provisional Patent Publication (Kokai) No. 58-20950 (hereinafter called "Conventional System 2").
According to Conventional System 1, the air-fuel ratio is controlled by an automatic choke valve when the ambient temperature of an internal combustion engine is below a first predetermined value, by an air-fuel ratio control valve which regulates an amount of air introduced into an air bleed of a carburetor, in response to the ambient temperature when the ambient temperature is above the first predetermined value and below a second predetermined value, and by the air-fuel ratio control valve in response to the output from an exhaust gas sensor when the ambient temperature is above the second predetermined value, respectively. That is, according to Conventional System 1, when the engine ambient temperature is in an intermediate range between the first predetermined value and the second predetermined value, the air-fuel ratio is not controlled in feedback mode responsive to the output from the exhaust gas sensor but it is controlled in response to the engine ambient temperature by the air-fuel ratio control valve, which, however, results in the air-fuel ratio not being accurately controlled to a desired value. Further, when the engine ambient temperature is in the intermediate range, the air-fuel ratio has to be controlled to a relatively small or rich value required by the engine so as to prevent stalling of the engine which is being warmed up, which is disadvantageous in respect of fuel economy.
On the other hand, according to Conventional System 2, an O.sub.2 sensor which has an output characteristic linear with respect to the concentration of oxygen in exhaust gases is used as the exhaust gas sensor to sensor the actual air-fuel ratio, and the air-fuel ratio is controlled to a desired value in feedback manner such that an air-fuel ratio control valve which regulates the fuel supply amount is controlled in response to the result of comparison between the actual air fuel ratio and the desired one. However, if the air-fuel ratio control valve is controlled by means of digital computation, the resolution power of control of the valve is set as a function of a quotient resulting from equal division of the controlling range of the air-fuel ratio by a given number. Therefore, as the controlling range of the air-fuel ratio becomes larger, the resolution power becomes degraded. If Conventional System 2 is applied both during and after warming-up of the engine, the controlling range of the air-fuel ratio is larger as compared with the case where it is applied only after warming-up of the engine, and accordingly the resolution power, i.e. the control accuracy becomes degraded, thus making it difficult to control the air-fuel ratio to a desired value. After warming-up of the engine in particular, although the engine operation becomes stable so that the variation width of the air-fuel ratio becomes small, the air-fuel ratio has to be controlled in a fine manner so as to secure required driveability and exhaust emission characteristics of the engine, which, however, cannot be achieved owing to the above-mentioned degraded resolution power.